WASHINGTON — At a crucial juncture in special counsel Robert S. Mueller III's wide-ranging Russia investigation, President Donald Trump embarked on a risky gambit on Sunday, going on record to directly dispute his former FBI chief's sworn contention that the president had sought to derail an investigation into fired national security adviser Michael Flynn.

The imbroglio — set off, not surprisingly, by a presidential tweet — comes on the heels of Flynn's guilty plea to charges of having lied to the FBI in connection with conversations with Russia's then-ambassador to Washington during the presidential transition.

Friday's dramatic development was met with initial silence from Trump, then with a deluge of weekend tweets in which he muddied the waters over his reasons for firing Flynn, excoriated the Justice Department and the FBI, renewed his attacks on rival Hillary Clinton and seemingly questioned the impartiality of Mueller's probe. He also explicitly contested statements by James Comey, who was fired seven months ago, regarding events prior to his dismissal.

Sunday's statement by Trump on Twitter was largely in line with his previous disparagement of Comey, whose truthfulness and even mental stability the president has questioned in the past.

Last month, the president told reporters traveling with him in Asia that Comey was a "liar" and a "leaker." In May, the day after firing the FBI director — an action that set in motion Mueller's appointment as special counsel — the president told senior Russian officials in an Oval Office meeting that Comey was a "crazy ... a real nut job," according to news reports based on transcripts of the encounter.

But the specificity and timing of the president's public denial of Comey's contention that Trump asked him to back off on investigating Flynn took on added significance with news that the former national security adviser is now cooperating with Mueller's investigation.

Speaking on CNN's "State of the Union," Warner said the fired FBI chief was "very credible in his testimony" when Comey went before the committee in June. The ex-FBI director has said he kept contemporaneous notes of an encounter in which Trump asked if he could ease off Flynn, characterizing the fired national security adviser as a "good guy."

Warner said he expected that Flynn being charged with only a single count of lying suggested that the former national security adviser had "many more stories ... to tell" about the Trump campaign and transition.

The response to major developments in the Mueller investigation has often diverged along partisan lines. But Flynn's guilty plea, which for the first time takes the probe inside the White House, drew cautionary language from some GOP lawmakers.

Sen. Tim Scott, R-S.C., also interviewed on CNN, said he agreed with those who said Trump should not pardon Flynn.

"We have to have a way to restore confidence of the American people in their elected officials and the leaders of this country," he told interviewer Jake Tapper. "One way you do that is by holding those folks who are ... lying to the FBI, you hold those folks accountable."

Against what would otherwise have been the triumphal backdrop of Trump's first major legislative victory, Senate approval of a major GOP tax overhaul plan, the current national security adviser, H.R. McMaster, was asked whether interactions with key allies were being colored by uncertainty over the fate of Trump's presidency due to the Mueller investigation.

"I don't think our allies need any reassurance," McMaster said on "Fox News Sunday," adding: "What we're doing is continuing to work with them on all the key challenges we face today," including North Korea and the fight against Islamic State.

McMaster declined to characterize any personal feelings over his predecessor, a military man like himself, having admitted to lying to the FBI.

"My thoughts are on what we have to do for the country," he said. "I don't really have a lot of spare time to think about other topics, other than our greatest national security challenges."

Trump's senior aides have acknowledged their inability to rein him in on Twitter, but the president's penchant for off-the-cuff observations could have serious implications for his own legal standing in the ongoing Mueller investigation.

On Saturday night, Trump tweeted that he had been forced to fire Flynn because the former Army lieutenant general had lied to Vice President Mike Pence about his conversations with Sergey Kislyak, then Russia's envoy to the U.S. But Trump also blamed the firing on Flynn having lied to the FBI, which the White House had not previously acknowledged knowing at that point in time. One of Trump's frequent critics, former Office of Government Ethics head Walter Shaub, seized on Trump's tweet, asking in his own tweet, "Are you ADMITTING you knew Flynn had lied to the FBI when you asked Comey to back off Flynn?"

A number of legal experts said such an admission by Trump could expose him to accusations of obstructing justice.

Seeking to defuse the controversy, the president's personal lawyer, John Dowd, told ABC News that he drafted the tweet on Flynn's dismissal, characterizing it as "sloppy."

The apparent walking back of Trump's statement did not satisfy some Democrats. Sen. Dianne Feinstein, D-Calif., interviewed Sunday on NBC's "Meet the Press," said "what we're beginning to see is the putting together of a case of obstruction of justice" in connection with the firing of Comey and other actions.

In a series of tweets on Sunday, Trump also sought to cast doubt on the impartiality of Mueller's investigation, citing reports that FBI agent Peter Strzok had been removed from the special counsel's team after an internal investigation of text messages he reportedly wrote that were interpreted as being critical of Trump. Mueller's office confirmed that he had been reassigned in late July.

Trump declared that the FBI's reputation was "in Tatters, worst in History!" in the wake of Comey's tenure. "Fear not," he wrote. "We will bring it back to greatness."

In response Sunday, Comey, who in recent days has been tweeting famous quotations about truth and justice, offered a quote from his own Senate testimony earlier this year. "I want the American people to know this truth: The FBI is honest. The FBI is strong. And the FBI is, and always will be, independent."

The president's Sunday tweets also included voicing approval for a decision by ABC News to suspend journalist Brian Ross for erroneously reporting that Trump directed contacts with the Russians as a candidate, not as president-elect.

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